1. Field of the Invention
The invention relates to digital imaging devices and more particularly to digital pixel configuration.
2. Description of Related Art
Imaging sensing devices are the light detecting component in digital imaging systems, such as for example, digital cameras. An image sensing device, such as a camera, uses light to capture an image by a semiconductor-based chip. The chip replaces film in traditional film-based systems. In a camera, an image sensing device is configured, in its simplest form, to capture a monochrome or color image by way of semiconductor devices such as transistors, capacitors, and photodiodes. In one example, the image sensing device is a chip made up of a number of pixels, each pixel capable of absorbing light. In color applications, each pixel generally absorbs light through a filter and represents one color corresponding to the image sensed.
In general, a pixel contains a photosensing structure, such as a photodiode, and other pixel circuitry. The photosensing structure is the region of the pixel that responds to light. For example, a pixel circuit having a photodiode is charged to, for example, 5 volts. The photodiode is exposed to light and the pixel circuit discharges its stored energy depending on the intensity of the light exposure.
FIG. 1 illustrates an example of a prior art pixel circuit 10 described in U.S. Pat. No. 5,471,515. Prior art pixel circuit 10 contains a photosensing structure 20, such as for example, a photodiode that is sensitive to light. The circuit also contains a SAMPLE capacitor 70. SAMPLE capacitor 70 is initially charged to, for example, 5 volts by connecting the circuit to V.sub.CCT 50 which is the source connection of transistor M1 through transistor M2. Pixel circuit 10 also includes a RESET signal 30 that is coupled to the gate of transistor M1 that charges diode 20 when desired. Pixel circuit 10 contains a SAMPLE signal 40 coupled to the gate of transistor M2. SAMPLE signal 40 regulates the amount of time that photodiode 20 is permitted to be exposed. In other words, SAMPLE signal 40 regulates the discharge time of capacitor 70 according to, for example, the light exposure. Pixel circuit 10 further contains a driver, represented by transistor M3, which receives power from V.sub.CC 60 to take the signal from pixel circuit 10 and drive it off the chip to other process circuitry. Transistor M4 selects when transistor M3 drives bitline signal 90 which signals pixel 10.
A digital imaging device is made up of a plurality of pixels. One problem in using a plurality of semiconductor devices to make the individual pixels is that each of the pixels across the chip can differ. This is particularly seen in the amount of underlying leakage occasioned by the circuitry associated with charging and storing the individual capacitors associated with each pixel circuit. The leakage problem is addressed by compensating for the leakage on a pixel by pixel basis. This is done by conducting a dark picture operation wherein capacitor 70 is charged by V.sub.CCT 50 through transistors M1 and M2. Transistor M2 is turned off and the dark-picture reference signal in capacitor 70 is driven out. Through this operation, the device can compensate for individual leakage by each pixel by evaluating each pixel individually. Thus, the steps to, for example, take a picture include taking a picture of the desired image and then taking a dark-picture to calibrate the individual pixels.
FIG. 2 illustrates a schematic layout of a pixel array 100 for a digital imaging device, such as for example, a digital camera. FIG. 2 shows six pixels 110, each pixel being substantially square in shape and having individual pixel circuitry 10, such as the circuitry described in FIG. 1. FIG. 2 also shows pixel circuitry 10 coupled to photosensing structure 20, such as for example, photodiode 20 in FIG. 1. As shown in FIG. 2, the individual pixels 110 that make up the pixel array 100 are formed substantially identically or replicated throughout pixel array 100.
In digital imaging devices, the portion of the pixel that absorbs light is very important because that portion comprehends or interprets the amount of light that hits the pixel. Thus, the larger the photosensing structure, such as a photodiode, is, the better the interpretation or comprehension of the light that hits the pixel.
Prior art structures are limited in the size of the photosensing structure by the amount of additional circuitry needed to operate the chip, such as pixel circuitry 10. Thus, FIG. 2 illustrates the typical configuration of a substantially square pixel having a photosensing structure 20 that consumes about 50 percent of pixel 110 surface area. The other 50 percent of pixel 110 cannot interpret any light that strikes that portion of the pixel.
Photosensing structure 20 is typically formed in the chip substrate. For a semiconductor substrate, such as a silicon substrate, a photosensing structure that is a photodiode is formed commonly in the chip via a P-N junction. The additional circuitry 10 is formed of transistors and contacts to the individual transistors. Thus, the contacts require conductive, generally metal, layers to the devices. These contacts are generally formed of aluminum or an aluminum alloy in a manner similar to other integrated circuit chips by the deposition of multiple conductive layers each separately insulated from one another by a layer of dielectric material.